Meeting Abstract
Metabolic rates are flexible traits that vary seasonally, but also among and within winters, with higher metabolic rates during colder periods. Seasonal variation in summit metabolic rates (Msum = maximum thermoregulatory metabolic rates) in birds is consistently correlated with changes in pectoralis muscle and heart masses and sometimes with variation in cellular metabolic intensity. To examine mechanisms of within-winter Msum variation, we examined the effects of short (ST, 0-7 d), medium (MT, 14-30 d) and long-term (30-yr means) temperature variables on pectoralis and heart masses, expression of myostatin and its proteinase activators TLL-1 and TLL-2, and activities of citrate synthase (CS; index of cellular metabolic intensity) in pectoralis and heart of house sparrows (Passer domesticus) and dark-eyed juncos (Junco hyemalis). For both species, pectoralis mass residuals were positively correlated with ST temperature variables, suggesting that cold resulted in increased catabolism of pectoralis muscle. Heart mass varied little with temperature in either species. Pectoralis mRNA and protein expression of the myostatin system was weakly correlated with ST and MT temperature variables for both species, consistent with the weak trends of ST temperature variables on muscle mass for both species. Pectoralis and heart CS activities were inconsistently correlated with temperature variables for both species, suggesting weak short-term regulation of cellular metabolic intensity for both tissues. Thus, neither muscle nor heart masses, their regulation by myostatin, nor cellular metabolic intensity varied consistently with winter temperature, suggesting that other factors regulate within-winter metabolic variation in these birds.
